Biol. Cell (2015) 107, 1–16 DOI: 10.1111/boc.201400077 Review Fibrillarin from Archaea to human Ulises Rodriguez-Corona*, Margarita Sobol†, Luis Carlos Rodriguez-Zapata‡, Pavel Hozak† and Enrique Castano*1 *Unidad de Bioquımica´ y Biologıa´ molecular de plantas, Centro de Investigacion´ Cientıfica´ de Yucatan,´ Colonia Chuburna´ de Hidalgo, Merida,´ Yucatan, Mexico, †Department of Biology of the Cell Nucleus, Institute of Molecular Genetics of the Academy of Sciences of the Czech Republic, Prague 14220, Czech Republic, and ‡Unidad de Biotecnologıa,´ Centro de Investigacion´ Cientıfica´ de Yucatan,´ Colonia Chuburna´ de Hidalgo, Merida,´ Yucatan, Mexico Fibrillarin is an essential protein that is well known as a molecular marker of transcriptionally active RNA polyme- rase I. Fibrillarin methyltransferase activity is the primary known source of methylation for more than 100 methylated sites involved in the first steps of preribosomal processing and required for structural ribosome stability. High expression levels of fibrillarin have been observed in several types of cancer cells, particularly when p53 levels are reduced, because p53 is a direct negative regulator of fibrillarin transcription. Here, we show fibrillarin domain conservation, structure and interacting molecules in different cellular processes as well as with several viral proteins during virus infection. Additional supporting information may be found in the online version of this article at the publisher’s web-site Introduction progression, senescence and biogenesis of small nu- The nucleolus is the largest visible structure inside clear RNA and tRNAs proliferation and many forms the cell nucleus. It exists both as a dynamic and sta- of stress response (Andersen et al., 2005; Hinsby ble region depending of the nature and amount of et al., 2006; Boisvert et al., 2007; Shaw and Brown, the molecules that it is made of. The main function 2012). Among nucleolar proteins (NOP), fibrillarin of this structure is ribosome biogenesis. This pro- is an essential protein that has been conserved in its cess involves transcription of rDNA, processing of sequence and function throughout evolution (Ochs rRNA and assembly of ribosomal proteins (Kressler et al., 1985; Jansen et al., 1991). Normally during et al., 1999). The nucleolus consists of three compo- interphase, fibrillarin can be detected in the transition nents: fibrillar centers (FCs), dense fibrillar compo- zone between FC and DFC, where rDNA transcrip- nent (DFC) and granular component. Over 4500 pro- tion occurs, and in the DFC, where the pre-rRNA teins were identified by multiple mass spectrometry processing takes place in eukaryotic cells (Ochs et al., and are involved in several cellular processes (Ahmad 1985; Sobol et al., 2013). Therefore, it is commonly et al., 2009). Besides ribosome biogenesis in recent used as a marker of active nucleoli. years, several other functions have been attributed Depending on the organism, fibrillarin mass ranges to the nucleolus, such as genetic silencing, cell cycle between 34 and 38 KDa and was originally described in the nucleolus of Physarum polycephalum (Chris- tensen et al., 1977). It is included in the superfamily of the Rossmann-fold S-adenosylmethionine (SAM) 1To whom correspondence should be addressed (email: methyltransferases (MTases) (Wang et al., 2000). [email protected]) The characteristics of this superfamily include a con- Key words: Cancer, Methylation, p53, Ribosomal biogenesis, RNA processing. Abbreviations: aFIB, Archaea fibrillarin; CBs, Cajal bodies; DFC, dense fibrillar served SAM-binding motif, the catalytic triad/tetrad component; FACT, facilitates chromatin transcription complex; FCs, fibrillar cen- [K-D-K-(H)] and seven-stranded β-sheet flanked by ters; GAR, glycine- and arginine-rich domain; MTase, methyltransferase; NOP1, α α β α nucleolar protein 1; NORs, nucleolar organiser regions; NS1, Non-structural pro- -helices to form an - - structure (Rakitina et al., tein; PIP2, phosphatidylinositol 4,5-bisphosphate; PNBs, pre-nucleolar bodies; 2011). Their primary and secondary structures are pol I, RNA polymerase I; PRMT1, protein arginine N-methyltransferase 1; RAP, conserved and one of their principal characteristics [RuLCl2]H4H2O; SAM, S-adenosylmethionine; ASF/SF2, alternative splicing factor/splicing factor 2; SMN, survival of motor neuron; snoRNA, small nucleo- is a site rich in arginine and glycine residues and a lar RNA; snoRNP, small nucleolar ribonucleoprotein; snRNA, small nuclear RNA; Tat, Trans-activator of transcription. specific motif to bind RNA. C 2015 Societ´ e´ Franc¸ aise des Microscopies and Societ´ e´ de Biologie Cellulaire de France. Published by John Wiley & Sons Ltd 1 U. Rodriguez-Corona and others Fibrillarin transfers the methyl group of SAM to not completely complement specific roles of eukary- 2-hydroxyl group of ribose target (Omer et al., 2002; otic fibrillarins. Furthermore, as in eukaryotic cells Ye et al., 2009). The MTase activity was confirmed fibrillarin is localised primarily in the nucleoli and by reconstruction of the small ribonucleoprotein Archaea do not have nucleus, it would be unlikely from Archaea Sulfobus solfataricus. Testing the recon- that all functions are conserved. Archaeal fibrillar- structed complex with mutations within the MTase ins have been used in recombinant protein purifica- domain of the Fbl gene helped confirm the methy- tion due to their high yield expression and ease to lation activity by Archaea fibrillarin (aFIB) (Omer remove other bacterial proteins with high tempera- et al., 2002). Recently, a new methylation activity has tures. Higher plant or vertebrate fibrillarins show a been attributed to fibrillarin. It mediates methylation very poor expression in bacteria in a native form. They of Gln-105 in histone H2A, which is a modification have to be purified from inclusion bodies and in most that impairs binding of the facilitates chromatin tran- cases refolded before biochemical experiments can be scription complex (FACT) complex and is specifically performed (Pearson et al., 1999). On the other hand, present at 35S ribosomal DNA locus, thus having an most of the genetic experiments were carried out epigenetic effect specific in active RNA polymerase I with NOP1 and detail localisation studies were done (RNA pol I) promoters. Abnormal levels of fibrillarin with vertebrate fibrillarins. It remains to be defined have been found in several types of cancers such as if the nonstandard interacting partners are the same breast cancer and prostate cancer (Koh et al., 2011; throughout the different kingdoms as well as under Miller et al., 2012) as well as interacting with viral the different stages of development and cell growth proteins from the Influenza A virus and the trans- conditions. One clear difference is that aFIB shows activator of transcription (Tat) protein from HIV (Yoo poor RNA binding (Omer et al., 2002) while fibril- et al., 2003; Melen et al., 2012). Here, we would like larin from different eukaryotic organisms shows well- to present a first view on what is known about this defined RNA binding activity and, in some cases, up protein and what still remains to be cleared. to two binding sites for RNA have been described (Rakitina et al., 2011). NOP1 function is essential for the modification Fibrillarin phylogenetics and processing of pre-rRNA. NOP1 can be replaced The term ‘fibrillarin’ has been used indistinctly for by the fibrillarin of Arabidopsis thaliana (Barneche several proteins from many organisms; in particular, et al., 2000) as well as by human and Xenopus fibril- aFIBs that have significant differences and could con- larins (Schimmang et al., 1989; Jansen et al., 1991). fuse newcomers. Moreover, several synonyms exist in However, the protozoan fibrillarin from Tetrahymena the literature such as 34 kDa nucleolar scleroderma termophila does not complement yeast counterpart, antigen, Dmel CG9888, CG9888, Dmel CG9888ri, possibly due to the differences in the amino termi- GCR-6, GCR6, Pen59C5, fib, pen59C5, Fib, FIB, nal domain. A domain that is rich in glycine and FBL, Fbl, FIB1, FLRN, RNU3IP1, rRNA 2-O- arginine residues (termed the GAR domain) and has methyltransferase fibrillarin, NOP1, nop1, fibM and a low sequence similarity when compared between aFIB depending on the organism and the time when different organisms (David et al., 1997). This can the reference was published. Here, to avoid confusion suggest lower general conservation that is commonly and to distinguish between different organisms, we believed. Moreover, replacement of NOP1 by hu- will use the term ‘fibrillarin’ for all eukaryotic fibril- man, Xenopus or A. thaliana fibrillarin changes the larins with the exception of yeast fibrillarin (NOP1) growth and nuclear morphology in yeast, thus show- and define the data from the individual organisms ing that not all fibrillarin functions are conserved by adding genus and species before the term when (Jansen et al., 1993). the observations could be unique. We will use the The sequence alignments and comparison of 10 term aFIB for all archaeal organisms as used by Omer model eukaryotic fibrillarins and all aFIBs was carried et al. (2002). We think it is particularly important to (Figure 1A). Archaea bacteria present a large spec- make this distinction considering the large amount trum of cladograms that separate from other groups. of biochemical and structural data that has been ob- The sequence comparison for all complete eukaryotic tained from archaeal organisms whose aFIB could fibrillarins is included (Figure